1. Field of the Invention
The invention relates to a device for stabilizing the reflectivity of a phase conjugation mirror. This device is applicable to improving the reflectivity of mirrors having phase conjugation through stimulated Brillouin scattering at high pulse repetition rate.
2. Discussion of Background
Stimulated Brillouin scattering is a physical mechanism well suited to the conjugation of the phase of short pulses within the 1-10 ns range of duration. Applications relate to the correction of optical phase distortion from various causes, for example within solid-state laser media such as YAG:Nd. A geometry very often used to produce such phase conjugation mirrors is represented in FIG. 1: the incident wave which has suffered aberration is focused in the non-linear medium, which is most often a gas (CH.sub.4, Xe, . . . ) or a liquid (acetone . . . ). Beyond a peak power threshold, the creation is observed of a reflected pulse, with a high efficiency (up to 90%) which is phase conjugated with the incident wave.
If this performance is achieved in the case of single incident pulses, nevertheless the fact of raising the repetition rate leads to a drop or to fluctuations in reflectivity as a function of time, which is correlated with a degradation in the quality of the phase conjugation (fidelity). These harmful effects are certainly due to the heating of the non-linear medium as a result of its non-zero absorption, which leads to the formation of turbulences which degrade the process.
Several solutions have already been envisaged for overcoming this reduction in performance at high rate. These solutions provide for circulating the fluid in the gas cell so as to renew it in the interaction zone with each pulse. This solution requires the production of laminar fluxes in the interaction region and complicates the use of phase conjugation mirrors in systems where the size constraints are severe.